U.S. Pat. No. 4,423,349 to Nakajima et al discloses two methods of making the above phosphor. In one method, carbonates of lanthanum, cerium and terbium are formed at 70.degree. C. and then reacted with phosphoric acid at 75.degree. C. to form a lanthanum cerium terbium phosphate. The phosphate is then fired at 1150.degree. C. for 75 minutes to form the phosphor.
In the other method, a coprecipitate of lanthanum cerium terbium oxalate is formed at 80.degree. C. and decomposed at 800.degree. C. to form a single-phase, mixed valence state lanthanum cerium terbium oxide. Diammonium phosphate is added to the oxide, and the mixture is then decomposed at 700.degree. C. to form a lanthanum cerium terbium phosphate. The phosphate is then crushed and fired at 1200.degree. C. for 4 hours to form the phosphor. Boron oxide or ammonium borate as a fluxing agent may be added before the firing step to promote the reaction and improve brightness.